What is Ohmmeter? Construction, Circuit Diagram, Types & Working

Ohmmeter is used to directly measure resistance of a device, element, circuit or any portion thereof. This meter is used to determine approximate (not too accurate) resistance of the circuit components. It can be used to measure the resistance of heating element of heater, electric iron, field coil resistance, sorting resistors used in electronics circuits. It can also be used for checking semiconductor diodes. For checking continuity of circuits ohmmeter is useful. In balancing the bridge to save time the ohmmeter is helpful in the laboratories to know approximate value of resistance i.e. helpful in bridge balancing.

Types of Ohmmeter

There are two types of Ohmmeter,

1. Series Type Ohmmeter
2. Shunt Type Ohmmeter

Series Type Ohmmeter

Figure 1: Ohmmeter.

For measuring the resistance, the unknown resistance is connected across terminals A and B of the circuit shown in Fig. 1. The circuit consists of a Battery, basic meter, R₂ shunting resistance and R₁ series resistance. Assembly of such items is shown in the Fig. 1. Shunting resistance (R₂) is in parallel to the meter resistance R current limiting resistance R₁ is in series as shown in the Fig. 1. Battery of emf E is also connected as shown in the Fig. 1. For measuring unknown resistance R_x is connected across A and B terminals for its measurement.

Now, say when R_x is not connected and terminals A and B are shorted, in such case maximum current flows through the meter. R₂ is adjusted till full scale deflection is indicated (see Figure 2). This position is marked as zero ‘0’ resistance. When terminals A and B are open then naturally resistance between A and B is ∞. So here on the scale ∞ is marked. Thus the span is 0 to ∞. The dial scale of series type Ohm meter zero on right side and ∞ left side. The meter is light in weight, compact and very easy to carry. Hence suitable for general service work. It measures the approximate (not exact) values of resistance. Care of Battery is to be taken. It discharges in repeated work or kept idle for a length of period.

Shunt Type Ohmmeter

 

The connections of battery E, adjusting resistance R₁, meter and a switch s are connected as shown in the Fig. 3. Terminals A and B for measuring unknown resistance when it is connected across them unknown resistance is marked as R_x. Switch ‘s’ is for disconnecting the battery when meter is not in use. When terminals A and B are shorted R_x = 0 meter current is zero. When A and B are open i.e. R_x = ∞. On the scale of this meter zero marking on the left and co on the right.

Applications of Ohmmeter

1. Used for measuring and sorting of resistors used in electronic circuits.
2. Used for checking of semiconductor diodes.
3. Used for checking continuity of circuits.